The present invention relates to a method of producing high-grade metal or alloy powder and, more particularly, to a method of producing powder of metals containing so-called active elements which exhibit high affinity to oxygen or nitrogen, such as Ti, Nb and Al and their alloys, as well as powders of metals generally referred to as refractory metals such as Mo, W, Ta and their alloys.
Hitherto, various methods have been proposed and actually carried out for producing powders of these metals and alloys.
In one of these known methods called as "Inert Gas Atomization Process", the metal is first molten in vacuum and is then atomized by an inert gas such as Ar, He and so forth. In another known method called as "Rotating Electrode Process", arc is discharged to a rotating electrode of the metal or alloy to physically atomize the molten metal by the centrifugal force. Some of these methods have been put into practical use.
Most of these known methods include a step of making a molten metal or alloy pass through a refractory nozzle and to drip the same into vacuum or an inert gas. The produced powder of metal or alloy, therefore, inevitably has inclusions due to contamination of the refractory material of the nozzle. In consequence, the product unavoidably suffers from various shortcomings or inconveniences which undesirably impairs the reliability of the product material as a structural material, such as reduction in the low-cycle fatigue strength triggered by these inclusions. The aforementioned rotating electrode process, which does not necessitate any nozzle, has been proposed to overcome the above-mentioned shortcomings or inconveniences. This process, however, still suffers from disadvantages such as contamination of the product by W caused by a melting of the W electrode which cooperates with the rotating electrode in making the arc discharge therebetween. In order to overcome this problem, a method called as "Plasma Rotating Electrode Process" has been proposed as a modification of the rotating electrode process, in which a plasma jet of He gas is applied to the rotating electrode of the metal or alloy. This method, however, has not been put into practical use due to a problem of entrapment of ionized He gas by the product metal or alloy powder, because the application of the plasma jet is effected in the He gas atmosphere of substantially atmospheric pressure to permit the ionization of the He gas by the plasma. In addition, the methods employing the rotating electrode require solid electrode material having a superior quality without any defect such as shrinkage cavities, in order that the solid electrode material withstands a high mechanical tensile force applied by the centrifugal force during the operation rotating at high speed. The production of the solid electrode of such a superior quality necessitates various steps such as forging, rolling and so forth to impose a problem from the view point of economy.
Thus, all of the methods and apparatus for producing metal or alloy powder proposed and used hitherto involves various problems or drawbacks, which constitutes a bottleneck in spreading the use of powder of superalloys and Ti alloy.